Magnesium deficiency is very distinctive and easy to diagnose. The lower leaves turn very light yellow, except for the veins which remain green. The sick leaf edges also feel crispy.
Magnesium is a mobile element. Plants can take it from older growth to save new growth. Symptoms will show up in the old growth first.
Symptoms appearing on the old growth first is the way to distinguish a magnesium deficiency from an iron deficiency which has the exact same symptoms except they appear on the new growth first.
As the deficiency gets worse, symptoms travel up the plant, finally affecting the leaf shoots which change from healthy green to golden.
Magnesium deficient plants respond very quickly to Cal-Mag added to the reservoir, or applied as a foliar spray. It’s the easiest deficiency to fix.